Although Kaptchuk, an associate professor of medicine, has spent his career studying these mysterious human reactions, he doesn’t argue that you can simply “think yourself better.” “Sham treatment won’t shrink tumors or cure viruses,” he says.

But researchers have found that placebo treatments—interventions with no active drug ingredients—can stimulate real physiological responses, from changes in heart rate and blood pressure to chemical activity in the brain, in cases involving pain, depression, anxiety, fatigue, and even some symptoms of Parkinson’s.